Make undo-tree possible to disable

[ryukinix]

Is your feature request related to a problem? Please describe.

I'm considering replacing undo-tree with vundo due performance issues.

Describe the solution you'd like

Make possible to disable undo-tree, which is enabled by default.

Describe alternatives you've considered

Create a variable to toggle if undo-tree should be enabled, around these lines:

https://github.com/bbatsov/prelude/blob/master/core%2Fprelude-editor.el#L361-L368

It can be enabled by default to be a backward compatible change, but it should be possible to customize on user personal config.

[ryukinix]

I can do this if there is no objections.